LITTLETON, Colo. — Schools in the Denver area are on heightened alert after authorities said a young woman who is "infatuated" with the 1999 shooting at Columbine High School made threats just days before the 20th anniversary of the attack that killed 13 people.

Authorities are looking for Sol Pais, 18, who is thought to have made undisclosed threats that prompted Columbine and more than 20 other schools outside Denver to lock their doors for nearly three hours Tuesday afternoon.

All schools in the Denver area were urged to tighten security because the threat was deemed "credible and general," said Patricia Billinger, a spokeswoman for the Colorado Department of Public Safety.

Some schools released their students after additional security was called in and canceled evening activities or moved them inside.

"We always have heightened awareness close to high-profile anniversaries like this," Billinger said.

Pais traveled to Colorado from Miami on Monday night and bought a pump-action shotgun and ammunition, according to the Jefferson County Sheriff's Office and the FBI. She was last seen in the foothills west of Denver, was considered armed and extremely dangerous and should not be approached.

"This has become a massive manhunt ... and every law enforcement agency is participating and helping in this effort," Dean Phillips, special agent in charge of the FBI in Denver, said late Tuesday night.

The FBI's Rocky Mountain Safe Streets Task Force issued a notice Tuesday describing Pais as "infatuated with (the) Columbine school shooting." The alert also said police who come into contact with her should detain her and evaluate her mental health.

Sheriff's spokesman Mike Taplin said the threats she made were general and not specific to any school.

The Denver Post reported that a call to a phone number listed for Pais' parents in Surfside, Florida, was interrupted by a man who identified himself as an FBI agent and said he was interviewing them.

The Associated Press left messages at two numbers listed for Pais' relatives in Florida, while another number was disconnected.

The doors were locked at Columbine and more than 20 other schools in the Denver area as the sheriff's office said it was investigating threats against schools related to an FBI investigation.

Columbine students continued attending classes in the afternoon and left school on time, but after-school activities were canceled on the campus in Littleton, Colorado.

Two teenage gunmen attacked Columbine on April 20, 1999, killing 12 classmates and a teacher.